The present invention relates to a measuring device to be mounted on a vehicle, such as automobile, and to measure distances from various targets around an own vehicle and also measure relative speeds between them, and particularly to a distance measuring apparatus to realize a high axial resolution by a simple hardware.
A millimeter-wave radar to be used for the automobile radiates a radio wave of a millimeter-wave band, receives reflected waves from the targets such as other vehicles and obstacles, and detects a propagated time period of the waves, an intensity of the reflected waves, a Doppler shift amount of frequencies, etc. The distance from the target and the relative speed between the own vehicle and the target are then measured from a result of the detection. There are several methods to measure the distance and srelative speed.
Japanese Patent No. 3203600 discloses a dual-frequency CW (Continuous Wave) method which is a typical method of the millimeter-wave radar used for the automobiles. The dual-frequency CW radar measures a relative speed of a target in accordance with the Doppler shift at a reception frequency to then measure a distance as far as the target in accordance with phase information of reception signals at the two frequencies.
A principle of the dual-frequency CW method will be described with reference to FIGS. 18A to 18C. In the case of using a single transmitter, two frequencies f1 and f2 are transmitted while they are switched alternately, as shown in FIG. 18A. In the case of a related technique of the radar for use in the automobiles, a frequency for switching periodically the frequencies f1 and f2 is about 100 Hz, and a difference fdev between the two frequencies f1 and f2 is about 300 kHz.
In a reception side, reception signals at the respective transmission frequencies f1, f2 are subject to a fast Fourier transformation (FFT) processing to obtain frequency spectra of reception beat signals.
FIG. 18B shows an example of a reception frequency spectrum. A signal (target information) as shown in FIG. 18B emerges at a frequency (Doppler frequency) corresponding to a relative speed of the target on the frequency spectrum, when the target emerges.
In the case of the dual-frequency CW method as a radar system using the Doppler frequency, target information can be obtained from the two transmission frequencies f1, f2. It is therefore possible to separate and detect a plurality of targets each having a different relative speed from the foregoing obtained information of the frequency spectra, and a relative speed “Rate” for each of the separated and detected targets is calculated from the Doppler frequency ft by using the following expression (1).
                    Rate        =                              ft            ·            c                                2            ⁢            fc                                              (        1        )            
where fc is a transmission frequency, and c is the velocity of light.
FIG. 18C shows a vector representation of a phase and amplitude in frequency spectrum information at two reception signals in the dual-frequency CW method. Here, a phase angle difference θ between two power spectrums F1, F2 is proportional to a distance as far as the target.
Assuming that the power spectrums F1, F2 are represented by complex numbers Signal (1), Signal (2), the distance “Range” is calculated by the following expression (2) in accordance with the phase difference θ between the two frequencies since the difference fdev between the transmission frequencies f1, f2 is known.
                    Range        =                              c            ×            θ                                4            ⁢            π            ×                          f              dev                                                          (        2        )            
where fdev=f2−f1, θ=arg(Signal (1))−arg(Signal (2)), and c is the velocity of light.
As described above, the radar using the dual-frequency CW method calculates the relative speed of the target in accordance with the Doppler frequency to then calculate the distance as far as the target in accordance with the phase angle.
Japanese Patent No. 3746235 relates to a distance measuring apparatus which utilizes a Doppler shift of a reflected wave from a target under measurement to separate a plurality of targets and to detect each of the targets. The foregoing distance measuring apparatus radiates a radio wave, receives a reflected wave from the target, and detects the target, in which the apparatus includes: a transmitting unit that transmits continuously a first frequency signal for a predetermined time period or more, transmits continuously a second frequency signal having a predetermined frequency difference from the first frequency, and transmits a signal having a frequency difference of an integer multiple equal to or greater than twice the predetermined frequency difference from the first frequency over signals at N frequencies, where N is an integer equal to or greater than one; a receiving unit that measures a Doppler frequency of the reflected wave from the target at each of the respective transmission frequencies of the first frequency signal, second frequency signal, and N frequency signals; and a detection processing unit that separates the plurality of targets to then detect each of the targets.
In the case of the radar apparatus utilizing the foregoing Doppler shift, there is a problem that the Doppler frequency is tuned into zero to cause a target not to be detected, in a condition where the distance between the radar and target is not changed in time, that is, the relative speed is zero.
Further, in the case where plural targets emerge at the same speed, there is also a problem that it is difficult to separate the targets and detect each of the distances in high accuracy.